High-reliability air brake system providing a plurality of operational modes

ABSTRACT

An air brake control system of high reliability is provided for a vehicle of the type having front wheel brakes, rear wheel brakes, a first brake control element such as a pedal which actuates the front and rear brakes, and a second brake control element such as a pedal which also actuates the front and rear brakes. First and second pneumatic circuit means for controlling the brakes are duplicated back to the outlet of the air compressor in an arrangement where one side of the circuit is automatically isolated from the other to maintain braking capacity if a loss of air pressure occurs on the one side. Means is associated with air streams of only a selected one of the first and second means for providing a function absent in said first and second means.

This is a division of application Ser. No. 695,879 filed June 14, 1976,now U.S. Pat. No. 4,063,624.

BACKGROUND OF THE INVENTION

This invention relates to braking systems for vehicles and moreparticularly to air brake systems for vehicles which have a plurality ofbraking devices and a plurality of separate controls which the operatormay manipulate to initiate any of a plurality of different forms ofbraking.

Many vehicles have a plurality of sets of brakes, partly to provide thesafety of redundancy and in part to enable selection of any of a numberof different modes of braking. A wheel loader vehicle, for example, ofthe type having an elevatable bucket for scooping up and lifting bulkmaterials typically has a set of front wheel brakes, a set of rear wheelbrakes and a parking and emergency brake. Such vehicles are oftenequipped with a first brake pedal or the like which applies the frontand rear wheel brakes for slowing and precisely controlling vehiclemotion and may also have a second brake pedal which, in addition toapplying the front and rear brakes, temporarily establishes a neutralcondition in the vehicle transmission to facilitate bringing the vehicleto a full stop. Still another control activates the parking andemergency brake which in air-operated systems is usually spring-biasedto the engaged position so that it goes on automatically if controlsystem pressure should be lost.

It has heretofore been the practice to provide some degree of redundancyor duplication of pneumatic circuit elements which control the brakes sothat if a pressure loss occurs in one side of the system, from ruptureof a hose or fitting or other cause, at least some of the normalelective braking capacity is retained. In braking systems of the kindoutlined above, this duplication has not been as complete as would bedesirable.

Considering another factor very significant to the reliability of an airbrake system, the possibility of malfunction from pressure loss isrelated to the number of fittings and connections between scatteredcircuit components that are required in the system. In certain priorforms of air brake system, a number of the circuit components have beensituated within a single housing assembly which arrangement has theeffect of reducing the number of fittings and connections in thecircuit. These prior arrangements do not provide the control functionsrequired in a multimode braking system of the particular kind describedabove and are not readily susceptible to modifications to accommodate tothe needs of such a system.

SUMMARY OF THE INVENTION

This invention is an air brake control system having a simple andeconomical construction that provides for high reliability of brakingfunctions in a vehicle having front brakes, rear brakes and controlcircuit arrangements. More particularly, the system is suited for avehicle having a first brake control element for controllably applyingthe front and rear brakes through a first means, a second brake controlelement for also applying the front and rear brakes through a secondmeans. Means is associated with air streams of only a selected one ofthe first and second means for providing a function absent in said 1stand second means.

The invention, together with further objects and advantages thereof willbe better understood by reference to the following description ofpreferred embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings

FIG. 1 is a schematic diagram of a vehicle braking system embodying theinvention,

FIG. 2 is a section view of a first modular valve assembly which may beemployed in the braking system of FIG. 1 to advantageously unitizeseveral of the pneumatic elements of the circuit into a single assembly,

FIG. 3 is a section view of the valve assembly of FIG. 2 taken alongline III--III thereof,

FIG. 4 is a section view of another modular valve assembly of the systemof FIG. 1, and

FIG. 5 is a schematic diagram of a modified form of the valve assemblyof FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIG. 1 of the drawings, an air brake system 11 isshown in schematic form together with certain components of a vehicle 12with which the brake system most directly coacts. The brake system wasdesigned for use on a wheel loader vehicle of the kind having anelevatable bucket for lifting earth or other bulk materials but it willbe apparent that the invention is equally applicable to certain otherforms of vehicle which have dual brake pedals or the like for initiatingmore than one mode of braking. As the vehicle 12 may itself be of knownconstruction, only certain components of the vehicle are shown, in blockform, in order to facilitate an understanding of the coaction of thepresent invention with other vehicle elements.

Vehicles of the type of which the invention is applicable are equippedwith a set of front brakes 13 at the front wheels and a set of rearbrakes 14 at the rear wheels and are usually additionally equipped withstill another separate parking and emergency brake 16. The front andrear wheel brakes 13 and 14 are typically of the form which areself-biased to a disengaged position but which engage when air or othergas under pressure is applied to inlet ports 13' and 14' of the oppositeform which is held engaged by internal spring means or the like exceptwhen air under pressure is applied to an inlet port 16'. Utilizing aparking and emergency brake of this spring-engaged kind adds to failsafety of the system since if air pressure should fail in the controlsystem for any causes, then the vehicle is automatically braked by theresultant loss of air pressure at port 16'.

In order to prevent the vehicle engine 17 from working against the frontand rear brakes 13 and 14 when the brakes are being applied, vehicles ofthe type to which the system is applicable often include a transmissionneutralizer 18 which functions to temporarily place the vehicletransmission in neutral, regardless of the setting of the transmissionshift control lever, when pressurized air is applied to an inlet port18'. As will hereinafter be discussed in more detail, the transmissionneutralizer 18 may be pressurized at the same time that the front andrear brakes 13 or 14 are pressurized and therefore acts to decouple thevehicle engine from the wheels during the period of braking.

For somewhat similar reasons, such vehicles may also be equipped with atransmission lockout device 19 which acts to urge the transmission shiftlever to the neutral position except when an inlet port 19' ispressurized when the parking brake 16 is actuated, by means which willhereinafter be described, to inhibit inadvertent shifting of the vehicleinto a drive setting of transmission while the parking brake is engaged.

Suitable detailed constructions for each of the above-described airpressure controlled vehicle components, specifically the front and rearbrakes 13 and 14, parking and emergency brake 16, transmissionneutralizer 18 and transmission lockout device 19, are known in the artand accordingly will not be further described.

Considering now the pneumatic circuit of the brake system 11, vehiclesof the type to which the invention is applicable are customarilyequipped with two separate brake pedals 21L and 21R or other equivalentcontrol elements which may be separately manipulated by the vehicleoperator to initiate a selected one of two different modes of brakingoperation. In particular, actuation of one control, such as the leftpedal 21L, normally actuates both the front and rear brakes 13 and 14simultaneously actuates the transmission neutralizer 18. This first modeof operation, initiated by operation of left pedal 21L in particular, isused when it is desired to bring the vehicle to a full stop or to effecta very large reduction of vehicle speed. Under those conditions it isnot desirable that the engine be working against the brakes andactuation of the transmission neutralizer 18 keeps this from occurring.

The alternate mode of operation is initiated by actuating the other orright brake pedal 21R. In this alternate mode, the front and rear brakes13 and 14 are actuated without neutralizing the transmission. This othermode of braking is used where it is desired to slow or retard motion ofthe vehicle a relatively small amount without bringing the vehicle to afull stop and without interrupting drive to the wheels.

Parking and emergency braking can be initiated by the operator by meansof an independent control element, which is a translatable knob 22 inthis example, that may be pulled to actuate the parking and emergencybrake 16.

To provide pressurized air for actuating the brakes and otherair-operated components described above, the vehicle is normallyequipped with a compressor 23 driven by the vehicle engine 17.

Failure of the braking system can be a hazardous condition in a vehicleand for this reason it is known practice to build at least some degreeof redundancy into a brake control system so that if a portion of thesystem should fail, other portions will continue to provide at leastsome braking capacity. The system depicted in FIG. 1 provides a veryhigh degree of such redundancy extending from the outlet of compressor23 to the inlet lines of the front and rear brakes 13 and 14 with aminimum of structural complication.

Basically, the control system 11 provides two parallel circuits foractuation of both the front brakes 13 and rear brakes 14 upon operationof either of the pedals 21L and 21R and further provides for operationof at least one of the sets of brakes 13 and 14 if air lines or otherpneumatic components associated with one such circuit should fail forany reason. This is additional to the fail safety provided by theparking and emergency brake 16 which, as previously pointed out,automatically engages if air pressure in the system should failcompletely.

Division of the control system 11 into two portions, each capable offunctioning independently of the other, begins at the compressor outletconduit 24 which is branched to couple to separate portions of apartitioned reservoir 26. In effect the two reservoir portionsconstitute a pair of separate air pressure reservoir vessels 26A and26B. Separate check valves 27A and 27B in the two branches of conduit 24block release of air pressure back to the compressor in the event ofcompressor failure or when the engine 17 is shut down.

Depression of the left brake pedal 21L by the operator opens each of apair of associated normally closed valves 28L and 29L. Similarly,depression of right brake pedal 21R opens each of another pair ofnormally closed valves 28R and 29R. Valves 28L and 28R share a singleair inlet port 31 which is communicated with reservoir 26A through anair conduit 32 while valves 29L and 29R share another single inlet port33 which is communicated with the other air reservoir 26B through aseparate air conduit 34.

Upon depression of the left brake pedal 21L, air from inlet port 31 istransmitted to an outlet line 36 of valve 28L while air from port 33 istransmitted to a separate outlet line 37 of valve 29L.

Outlet line 36 supplies such air pressure to the rear brakes 14 througha one-way check valve 38, which is bypassed by a flow restriction 39,and a shuttle valve 41 of the form having two inlets and a single outletand which functions as a resolver valve as will be hereinafterdescribed. For this purpose, the outlet of one-way check valve 38 isconnected to one of the two inlets of the shuttle valve 41 and theoutlet line 40 of the shuttle valve is communicated with rear brakeinlet port 14'.

Similarly, outlet line 37 responds to opening of valve 29L by supplyingpressurized air to the inlet port 13' of the front brakes 13 throughanother one-way check valve 42 and another shuttle valve 43 arrangedsimilarly to valves 38 and 41, the one-way check valve 42 again beingbypassed by a flow restriction 44 and the outlet line 45 of shuttlevalve 43 being connected inlet port 13' of the front brakes 13.

To actuate the transmission neutralizer 18 when brake pedal 21L isdepressed, still another shuttle valve 46 has opposite inletscommunicated with outlet lines 36 and 37 and has an outlet conduit 50communicated with transmission neturalizer inlet port 18'. Accordingly,transmission neutralizer 18 is actuated if either or both of outletlines 36 and 37 are pressurized.

Upon release of left brake pedal 21L, the front and rear brakes arevented, and therefore are disengaged, through shuttle valves 41 and 43,flow restrictions 39 and 44 and valves 28L and 29L. Transmissionneutralizer 18 is exhausted through shuttle valve 46 and valves 28L and29L. The flow restrictions 39 and 44 act to slow release of the brakesslightly to permit re-engagement of the transmission without abruptshocks.

For the purposes previously point out, a different mode of brakingoperation is initiated when the operator depresses the right pedal 21R.In this different mode, the front brakes 13 and rear brakes 14 are againapplied but the transmission neutralizer 18 is not actuated. For thispurpose the outlet line 47 of valve 28R is communicated with theremaining inlet of shuttle valve 41 and the outlet line 48 of valve 29Ris communicated with the remaining inlet of the other correspondingshuttle valve 43. Thus upon opening of valves 28R and 29R by operationof right pedal 21R, pressure is again delivered to the front and rearbrakes. Upon release of the pedal 21R, the front and rear brakes areexhausted by the valves 28R and 29R. Thus the mechanisms described aboveprovide for application of both the front and rear brakes in response tooperation of either of the pedals 21.

Considering now means for controlling the parking and emergency brake16, another shuttle valve 49 has one inlet coupled to air conduit 32 andthe other inlet is coupled to air conduit 34 to receive pressurized airfrom either reservoir 26A or 26B. Shuttle valve 49 has an outlet line 51connecting with an inlet of a parking and emergency brake control valve52. Control valve 52, which may be shifted by the operator of thevehicle by means of the previously described knob 22, has an outlet line53 communicated with the inlet port 16' of the parking and emergencybrake. As previously pointed out, brake 16 is of the form which isspring-biased to an actuated position to provide fail safety and is onlyreleased when pressurized air is applied to inlet port 16'. Accordingly,at the off position of know 22 control valve 52 transmits pressure fromsupply line 51 to outlet line 53 to release the parking and emergencybrake. Upon shifting of the control valve 52 to the brake-on position,inlet port 16' is disconnected from supply line 51 and vented at valve52 causing the parking and emergency brake to be actuated.

For the reasons previously discussed, inlet port 19' of the transmissionlockout 19 should be depressureized to engage the lockout when inletport 16' is depressurized to engage the parking and emergency brake 16.This may be accomplished by connecting transmission lockout inlet port19' with line 53 through another line 56.

Certain other vehicle components may be readily coupled into theabove-described system. For example, an air-operated horn 55 andactuator valve 57 may be connected into air conduit 32, for example, inproximity to one inlet of shuttle valve 49. Electrical componentsassociated with the braking system may also be coupled into the systemthrough pressure-actuated electrical switches. Such electricalcomponents are typically operated from the vehicle battery 58 throughthe operator's engine on-off switch 59 in the case of a diesel-poweredvehicle or through the ignition switch in the case of a gasoline-poweredvehicle. An audible alarm 61 and preferably an indicator light 62 aswell may be connected to switch 59 through both of two parallelconnected normally closed electrical switches 63 and 64 of the formwhich open in response to fluid pressure of a predetermined magnitude.Switch 63 may be communicated with air conduit 34 while theother switch64 is communicated with air conduit 32. Consequently, the alarm 61 andindicator light 62 are electrically actuated if system pressure dropsbelow a predetermined operating level at either or both of reservoirs26A and 26B or in the associated flow paths.

A parking brake "on" indicator lamp 66 may be controlled by anelectrical connection to engine switch 59 through another normallyclosed pressure switch 67 having a pneumatic pilot connection to theline 53 which communicates the parking brake with the parking brakecontrol valve 52 and which is held open by air pressure when it ispresent in line 53. The vehicle stop lights 68 may have parallelelectrical connections to engine switch 59 through a pair of normallyopen switches 69 and 71 which close in response to air pressure. Switch69 has a pneumatic pilot connection to the outlet of shuttle valve 43while the pneumatic pilot connection of switch 71 is coupled to theoutlet of shuttle valve 41. Thus, one or both of the switches 69 and 71are pneumatically closed, to energize stop light 68, when either or bothof the front brakes 13 and rear brakes 14 are actuated by air pressurefrom shuttle valves 43 and 41 respectively.

In operation, with air pressure reservoirs 26A and 26B pressurized andin the absence of system malfunction such as a ruptured air hose or thelike, the vehicle operator may selectively initiate any of three modesof braking. First, by depressing right pedal 21R, motion of the vehiclemay be retarded without interrupting drive from the engine to the wheelssince pressure is then controllably transmitted to both the front brakes13 and rear brakes 14 through the associated valves 28R, 29R and shuttlevalves 43 and 41. In a wheel loader vehicle for example this mode ofbraking is often used when the bucket is being driven into the pile ofmaterial to be loaded and a precisely controlled slow motion of thevehicle is desired. If the vehicle is to be brought to a full stop, theoperator normally utilizes the other brake pedal 21L as this not onlyactuates the front and rear brakes by transmitting pressure throughcheck valve 42 and shuttle valve 43 to the front brakes and bytransmitting pressure through check valve 38 and shuttle valve 41 to therear brakes but also, through shuttle valve 46, transmits pressure toinlet port 18' to activate the transmission neutralizer and therebytemporarily decouple the engine from the wheel drive line. Third, whenit is desired to park and immobilize the vehicle or for emergencybraking purposes in the event of malfunction of the other brakingsystems, the operator may activate parking and emergency brake 16 byshifting control valve 22 which then exhausts line 53 activating thespring-engaged parking brake and exhausts line 56 activating thetransmission lockout.

The foregoing description of the operation of the braking system isbased on the assumption that there is adequate air pressure in bothreservoirs 26A and 26B and that no significant system malfunction suchas a ruptured air hose, malfunctioning check valve or the like ispresent. An important characteristic of the described system is arelatively high degree of fail safety in that at least some brakingcapacity is retained in the event of any of various such malfunctions.It may be observed that while operation of either pedal 21R or 21Lnormally activates both the front brakes 13 and rear brakes 14, there isalso present, in effect, an alternate independent braking system foreach such set of brakes which is automatically established in the eventof loss of air in certain parts of the system. This division of thesystem into potentially independently operating systems for the frontand rear brakes extends all the way back through the reservoir 26 to theoutlet conduit 24 of the compressor.

Salient elements which constitute the potentially independent frontbrake actuating system include shuttle valve 43, check valve 42, rightand left pedal-operated valves 29R and 29L, air conduit 34 and reservoir26B. Corresponding elements of the potentially independent rear brakesystem include shuttle valve 41, check valve 38, right and leftpedal-operated valves 28R and 28L, air conduit 32 and reservoir 26A. Ifa pressure loss should occur, from hose rupture or excessive leakage, atany point in the system from the front and rear brakes back throughreservoirs 26, then operation of either brake pedal 21R, 21L willcontinue to actuate at least one set of brakes, the particular set whichremains actuatable being dependent on which portion of the system hasmalfunctioned. In the presence of such a malfunction, the shuttle valves41, 43, 46 and 49 act to seal off the malfunctioning side of the systemfrom the operative one. A loss of all braking capacity which is normallyavailable through operation of the foot pedals 21R and 21L would onlyoccur if both sides of the system malfunctioned simultaneously but thisdoes not mean that the operator would no longer have any electivebraking capacity. As long as pressure remains available at either sideof the system, emergency braking may be deliberately initiated byoperating valve 52. If there is a total loss of system pressure, theparking and emergency brake 16 is automatically actuated as previouslydescribed.

Most of the components of the above-described pneumatic circuitincluding the valves, flow junctions and the like may readily becontained in one or two unitized modular circuit component assemblies torealize the several advantages hereinbefore discussed. As previouslypointed out, such a construction increases system reliability byreducing the number of independent fittings and connections which may berequired, by over 50% in this example, inasmuch as such fittings andconnections are each a potential source of leakage. Modular unitizationalso improves serviceability by enabling location of at least a majorityof the valves, pressure switches and the like in one or two convenientaccessible locations on the vehicle. Cost of construction of the systemas a whole is also reduced by this means. Functionally distinct portionsof the system can, for example, be built up from structurally similarmodules in some cases.

In the present example, shuttle valves 41, 43, 46, check valves 38 and42 and flow restrictions 39 and 44, including the air flow pathjunctions between such elements are all unitized into a single modularassembly 72. Associated pressure controlled electrical switches, such aspressure switches 69 and 71, are also readily mounted on such a modularassembly 72. In this example, the shuttle valve 49 and associated flowjunctions are also contained within another unitized modular componentassembly 73 on which the other pressure switches 63 and 64 and 67 may bemounted. Although the second modular assembly 73 contains only a singleshuttle valve 49 in this example, the construction enables additionaloptional components to be more readily coupled into the system, anexample of which will hereinafter be described.

Considering now a suitable physical structure for the first modularassembly 72, reference should be made to FIGS. 2 and 3 in combination.Assembly 72 in this instance is comprised of two rectangular blockmodule bodies 74 and 76 secured together by suitable fasteners such asbolts 77. Module 74 has spaced-apart parallel flow passages 78 and 79 towhich the outlet lines 37 and 36 respectively from the left brake pedalcontrol valves are connected. Both such passages 78 and 79 extendtowards the surface of module 74 which is adjacent module 76 and bothsuch passages have enlargements 81 as such surface is approached. Toconstitute check valve 42, a disc 82 is disposed in the enlarged portion81 of bore 78 and to constitute check valve 38 a similar disc 83 isdisposed in the enlarged portion 81 of bore 79. Flow restrictions 44 and39 are defined by small central apertures in the discs 82 and 83respectively. The other module 76 has a first stepped flow passage 84adjacent to and continuous with the first passage enlargement 81 of thefirst module 74 and in which a compression spring 86 is disposed toextend between the two modules to bear against disc 82 and therebycomplete the check valve 42. Similarly, a second parallel steppedpassage 87 in second module 76 is aligned with the other enlargedpassage portion 81 of the first module 74 and contains a compressionspring 88 which bears against disc 83 to constitute part of the checkvalve 48.

To form the neutralizer shuttle valve 46, a relatively large bore 89extends in from one end of first module body 74, intersecting flowpassage 78 and has a reduced diameter end portion 90 communicated withflow passage 79. The outer portion of bore 89 including that portionwhich intersects flow passage 78 is plugged by a cylindrical member 91.Member 91 has an annular groove 92 which provides for flow passagecontinuity from line 37 to check valve 82 and the groove is alsocommunicated with the center of the innermost end of plug member 91 by apassage 93 in the member 91. The outlet line 50 of neutralizer shuttlevalve 46 communicates with bore 89 at a point equidistant from theadjacent ends of passages 90 and 93 and extends into the second modulebody 76 for connection to the transmission neutralizer inlet port aspreviously described.

Outlet lines 47 and 48 from the right brake pedal-operated valve andshuttle valve outlet lines 40 and 45 to the rear and front brakesrespectively each connect to the surface of second module body 76 whichis opposite from the first module body 74, and end member 96 beingsecured against module 76 by bolts 77 to receive the lines 48 and 47 inparticular. End member 96 defines one end of a cylindrical bore 97within the second module which is communicated with flow passage 84 atone end and with line 48 at the other end, the mid-region of bore 97being communicated with line 45 to the inlet port of the front brakes bya passage 45'. Bore 97, together with a cylindrical valve element 98which is slidable in an axial direction therein in response to airpressure, constitutes the front brake shuttle valve 43.

Similarly end member 96 defines one end of another cylindrical bore 99in second module body 76 with bore 99 being communicated at one end withthe outlet line 47 of the right brake pedal-controlled valve 28R andwith the other end of bore 99 being communicated with flow passage 87 ofthe second module. Line 40 to the inlet port of the rear brakes iscommunicated with the mid-region of bore 99 by a passsge 41'. Tocomplete the rear brake shuttle valve 41, a cylindrical valve element101 is disposed in bore 99 for axial movement therein in response to airpressures.

To inhibit leakage, resilient annular sealing elements 102, 103 and 104may be compressed between the two modules 74 and 76 in encirclingrelationship to one bore enlargement 81, flow conduit 50 and the otherbore enlargement 81 respectively. For similar reasons annular sealingelements 106 and 107 may be encircled around plug member 91 of the firstmodule at opposite sides of groove 92.

Considering now an advantageous aspect of the above-described modularassembly 72, additional ones of the component modules may be utilized todefine other functionally different portions of the circuit withoutsignificant modifications. Referring temporarily again to FIG. 1, it hasbeen pointed out that this example of the invention utilizes a separatemodular assembly 73 in addition to the one described above. However,this second modular assembly may in fact include a first module 74'which is structurally identical to the first module 74 of the modularassembly 72. A suitable physical structure for the second modularassembly 73 on this basis is depicted in FIG. 4.

As may be seen in FIG. 4, a third module 74' is identical in allstructural respects with the first module 74 of the other modularassembly 72 although in this case, it is air lines 32 and 34 whichconnect to the third module 74' and the internal shuttle valve of thethird module constitutes transmission lockout shuttle valve 49 of thesystem shown schematically in FIG. 1. The other module of modularassembly 73, that is the fourth module, may be a rectangular block 108secured against module 74' opposite from the side to which air lines 32and 34 connect. The fourth module body 108 contains parallelspaced-apart flow passages 109 and 111 which extend from the connectinglines 32 and 34 respectively to the flow passages 78' and 79'respectively of the module 74'. The several flow lines 51, 53, 53' and56 of the system as described with reference to FIG. 1 also connect tofourth module body 108. Referring again to FIG. 4, within module body108, a passage 112 connects line 51 with the outlet passage 113 fromshuttle valve 49 of the other module body 74' and another branchedpassage 114 in the module body 108 communicates line 53 with pressureswitch 67 and both of lines 53' and 56.

The modular construction readily lends itself to a convenient mountingof the previously described pressure-operated electrical switches on themodular assemblies. Thus the switches 63 and 64, which are electricallyconnected in parallel to operate an audible alarm and low air pressurewarning lamp as previously described, may be mounted on oppositesurfaces of module body 108 at openings 116 and 117 respectively whichcommunicate with internal flow passages 111 and 109 respectively.

Still another advantage of the modular construction is that it greatlyfacilitates the adding in of optional additional braking systemcomponents where these might be desired. Referring momentarily again toFIG. 1 it may be seen that release of air from the transmission lockout19 to actuate the transmission lockout occurs through the lines 56 and53 leading to the operator's parking and emergency brake control valve52. The arrangement shown is a preferred one in most instances but if,due to the configuration of a particular vehicle, these lines 56 and 53are necessarily very long, then release of the transmission lockout inresponse to opening of valve 52 may be undesirably slow. In that eventone of the above-described modules may be modified to include a relayvalve which provides a shorter exhaust path for the transmissionlockout. In particular and with reference to FIG. 5, there is shown amodified form of the modular assembly 73 which provides such means.

In the modified form 73' of FIG. 5, the third module 74' containing thetransmission lockout shuttle valve 49 remains unchanged and accordinglywill not be further described. The desired additional function isaccomplished by simply replacing the original additional module of theassembly with a modified fourth module 108'. As in the previousinstance, fourth module 108' connects the outlet passage 112' fromparking and emergency brake shuttle valve 49 with the air conduit 51 andalso provides a flow passage 114' connecting line 53 from the parkingand emergency brake control valve to line 53' which connects with theparking and emergency brake itself. The line 56 to the transmissionlockout also connects with module boyd 108' as in the previous instancebut in this case is not internally connected within module to line 114'.Instead, a relay valve 119 may be contained within module body 108'forthe purpose of directly coupling the transmission lockout controlline 56 to the outlet passage 112' of shuttle valve 49 in response tofluid pilot pressure in passage 114', which pilot pressure indicatesthat the parking and emergency brake is off.

For this purpose relay valve 119 may be of the two-positionpilot-operated form which is spring-biased towards a first position atwhich transmission lockout line 56 is connected to a vent or exhaustpassage 121 by the relay valve. The pilot of relay valve 119 isconnected to passage 114' by a pilot signal passage 122. When thepassage 114' is pressurized, indicating that the parking and emergencybrake is off, valve 119 is thereby piloted to the alternate position atwhich line 56 to the transmission lockout is disconnected from exhaustline 121 and receives pressure directly from outlet 112' of shuttlevalve 49.

It will be apparent that the arrangement of FIG. 5 is but one example ofhow supplementary or additional optional control system modificationsmay readily be provided by substituting in appropriately modifiedindividual modules.

While the invention has been disclosed with respect to certain preferredembodiments, it will be apparent that many modifications are possibleand it is not intended to limit the invention except as defined in thefollowing claims.

What is claimed is:
 1. An air brake control system for a vehicle havingan air compressor, front and rear brakes, and first and second brakecontrol elements, comprising:first and second air reservoirs eachconnected to the compressor; a pair of one-way flow valves, eachconnected to and between said compressor and a separate one of saidreservoirs; first means responsive to the first brake control elementfor controllably communicating an air stream from one of the airreservoirs with the front brake while communicating the rear brake withan air stream from the other of said reservoirs, said first meansincluding: a front brake shuttle valve having first and second inletsand an outlet, said outlet being connected to said front wheel brakes;first brake actuating valve means responsive to said first brake controlelement for controllably communicating the first inlet of said frontbrake shuttle valve with said first reservoir while separatelycommunicating the first inlet of said rear brake shuttle valve with saidsecond reservoir; second means responsive to the second brake controlelement for controllably communicating an air stream from one of the airreservoirs with the front brake while communicating the rear brake withan air stream from the other of said reservoirs, said second meansincluding: a rear brake shuttle valve having first and second inlets andan outlet, said outlet being connected to said rear wheel brakes; secondbrake actuating valve means responsive to said second brake controlelement for controllably communicating the second inlet of said frontbrake shuttle valve with said first reservoir while separatelycommunicating the second inlet of said rear brake shuttle valve withsaid second reservoir; and third means associated with air streams ofonly a selected one of the first and second means for providing afunction absent in said first and second means, said control systembeing of a construction sufficient for actuating one of the brakes andthe third means in the absence of an air stream between one of the airreservoirs and the other brake.
 2. A control system, as set forth inclaim 1, wherein the vehicle has a parking and emergency brake andincluding:a parking and emergency brake shuttle valve having first andsecond inlets and an outlet, said inlets each communicating with aseparate one of said reservoirs; a third brake control element; and athird brake actuating valve means responsive to said third brake controlelement for controllably pressurizing and venting said parking andemergency brake, said third brake actuating valve means being connectedbetween said parking and emergency brake and said outlet of said parkingand emergency brake shuttle valve.
 3. A modular circuit componentassembly for an air brake system comprising:first and second juxtaposedmodule bodies having adjacent surfaces and each having an oppositesurface; said first module body having a pair of parallel spaced apartfirst flow passages extending from said adjacent surfaces to saidopposite surface thereof and each having a diametrically enlargedregion, said first module body further having a second flow passageextending from a region of said adjacent surfaces situated between saidfirst flow passages to a different surface of said first module body,and still further having a pair of outlet passages each communicatingwith the central portion of said enlarged region of a separate one ofsaid first flow passages; said second module body having a pair ofparallel spaced apart third flow passages extending from said adjacentsurfaces to said opposite surface thereof in alignment with said firstflow passages, said second module body having a fourth flow passageextending between said pair of third flow passages and having adiametrically enlarged portion therebetween, said second module bodyhaving a fifth flow passage extending from the central portion of saidenlarged region of said fourth flow passage to said adjacent surfaces inalignment with said second flow passage; and first and second shuttlevalve members each being disposed in said enlarged region of a separateone of said first flow passages for axial movement therein, and a thirdshuttle valve member disposed in said enlarged region of said fourthflow passage for axial movement therein.